The present invention relates to wireless local area networks and, in particular, to an effective implementation of CSMA/CD technology in such environments.
Local area networks (LAN""s) are now commonplace and efforts to reduce their installation and upgrade costs have pursued the use of wireless technology, thereby facilitating affordable and convenient equipment placement and the like.
In normal wireless local area networks, collision detection is a difficult problem due to the so-called xe2x80x9chidden terminal problemxe2x80x9d. The impact of the hidden terminal problem is illustrated in FIG. 1 where A, B and C represent terminals and the circles represent the respective communication ranges of the terminals. When terminal A is communicating with terminal B (on the same frequency), terminal C cannot receive the signal from terminal A. Hence, terminal C might start to transmit to terminal B because terminal C is not aware of any signal being transmitted from terminal A. When this happens, the communication from terminal A to terminal B will be lost because of jamming induced by the signal transmitted from terminal C. In typical wireless networks, there are some common characteristics:
(a) the transceivers of mobile stations/terminals are generally low power (to conserve battery life), which generally limits their transmission range; and
(b) the transceivers of base stations can generally be of higher power because the power being consumed is traditionally from higher energy power supplies (ie. AC mains or large capacity batteries).
Hence, collision detection between coincident transmissions is almost impossible to prevent because of the hidden terminal problem.
Most existing methods of wireless local area networking are based on either TDMA (Time Division Multiple Access) or CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). Typical TDMA systems are exemplified by the products of Spectrix Corporation of the USA. The IEEE 802.11 wireless LAN standard uses a CSMA/CA method to contend for access to the medium.
The IEEE 802.3 standard, more commonly known as xe2x80x9cEthernetxe2x80x9d, has been known for more than a decade and is a widely implemented LAN standard in the wired networking domain. Ethernet is a multiple access technique based on CSMA/CD (ie. collision detection) because the wired domain is a very suitable environment for collision detection.
In the IEEE 802.11 standard, a terminal that has a data packet to transmit will first send a xe2x80x9cReady To Sendxe2x80x9d (RTS) control packet and then wait to receive a xe2x80x9cClear to Sendxe2x80x9d (CTS) control packet from the intended receiver terminal to ensure that the medium is free. If there is no CTS packet, the sender will perform an exponential back-off before attempting to again acquire access. Any other terminal in the communication range of either the sender or the receiver will hear either the RTS or CTS control packets and hence know that the medium will be busy for a period of time, and, as a consequence, those other terminals will not contend for the medium during this time. However, it has been found that the performance throughput of terminals at the edges of range of other terminals suffer because their RTS packets are usually not xe2x80x9cheardxe2x80x9d by the centrally positioned terminals. This is because the edge terminals cannot hear the terminals at the opposite edge that also transmit RTS packets towards the central terminals at the same time. Thus, when these terminals back-off, they will become synchronized in their attempts to transmit the RTS packet. Moreover, this method does not work when multicast addressed packets are attempted to be sent because there can be more than one receiver. Collision avoidance also suffers from the fact that different terminals can have different transmitter efficiencies due to varying power levels.
TDMA systems tend to be more complicated because of the timing requirements of the system. The base station needs to be xe2x80x9csmarterxe2x80x9d as they need to handle registration of new terminals that move within range of the base station.
CSMA/CD is acknowledged to be a simple and very efficient multiple access technique in wired networking, especially under heavy loads. CSMA/CD can achieve above 70% efficiency under heavy loads compared to CSMA/CA that can only achieve up to 30% efficiency under heavy loads. As explained earlier, the xe2x80x9chidden terminal problemxe2x80x9d makes the wireless environment unsuitable to implement CSMA/CD.
It is an object of the present invention to substantially overcome, or at least ameliorate, one or more problems with prior arrangements.
In accordance with one aspect of the present invention there is disclosed a wireless carrier sense multiple access/collision detection (CSMA/CD) communication system comprising:
a repeater station;
at least one terminal station;
a wireless uplink communication channel for one-way communications between each terminal station and the repeater station;
a wireless downlink communication channel for one-way communications between the repeater station and each terminal station, the uplink and downlink communication channels being operable at different transmission wavelengths, wherein a bit pattern received by the repeater station via the uplink channel is retransmitted via the downlink channel; and
a collision detection arrangement within each terminal station for comparing a bit pattern transmitted from the terminal station via the uplink channel with a bit pattern received from the repeater station via the downlink channel, to determine if the terminal station has unambiguous access to communications within the system.
In accordance with another aspect of the present invention there is disclosed a method of implementing a CSMA/CD communications protocol in a wireless network having a plurality of terminal stations, the method comprising the steps of:
providing a repeater station for receiving wireless communications on an uplink channel and retransmitting the same on a downlink channel having a communications wavelength different from the uplink channel;
enabling wireless communications for each terminal station via the uplink channel to the repeater station and to each terminal station from the repeater station via the downlink channel; and
performing collision detection within each terminal station by comparing a bit pattern transmitted by the terminal station with a bit pattern received by the terminal station such that if the two bit patterns coincide, no collision is detected.
Other aspects of the present invention are also disclosed.
The present invention acts to improve the wireless environment to an extent that will make it suitable to implement CSMA/CD. This improved environment allows the use of existing technologies based on the IEEE 802.3 Ethernet standard to implement efficient wireless LANs.
The use of the repeater station based on different transmitter and receiver wavelengths acts to solve the hidden terminal problem.